This research proposal is concerned with the immunological events of rejection of organ allografts and the mechanism of their prolonged survival by passive enhancement. We propose to expand and clarify previous experiments using organ allografts in the inbred rat to define more fully sequential cellular events in rejection and in enhancement. The cell population will be removed from cardiac allografts and from whole spleens transplanted in a graft-versus-host system at varying time intervals, and examined in regard to functional activity. The sub-population of small lymphocytes will be defined functionally by several methods. Macrophages will be removed and serial changes in their immunological status studied. The differential sequence of infiltration of the various effector cell population, and the variations in their function will be extensively investigated both in the unmodified host and in enhanced recipients of cardiac allografts. Immunofluorescence, rosetting techniques, and the Chromium 51 killer cell assay will be used. The self-limiting and spontaneously reversing rejection episode enhanced of heart grafts will be studied. The role of the spleen in the untreated animal and in those passively enhanced with alloantiserum will be investigated to define more fully its immunologic role.